Where does mkinitramfs get it's list of binaries and libraries to pack?

I've ported a product installer recently from Debian 10 to Ubuntu 20.04, and I'm having trouble getting the plymouth splash screens to show.

Tearing apart a good installer, and hijacking the install of 20.04, I can see that plymouth fails to show the splash screen because it couldn't find a suitable graphics device, which was due to the kernel modules missing. Fine, I add those to the modules file in initramfs-tools and they are now bundled in the initramfs file.

However, when the drm renderer is called, it fails due to to a missing library. I can see the libs in my good installer, but not in my new one. I can copy them individually in the build process, but I have to believe that mkinitramfs has the capability to determine the dependency and copy it for me. My overall view is that the dependency isn't copied in because I'm manually stuffing the /usr/lib/x86_64-linux-gnu/plymouth directory into the initrd directory after exploding the initrd file initially build by mkinitramfs.

How do I tell mkinitramfs to include those files/dirs? In order to troubleshoot the issue, I'd like some other binaries copied in as well, for instance less? How to I tell mkinitramfs which bin files to copy in?

Instead of copying the files to initramfs manually, you should create a custom initramfs script that uses copy_exec to place the files into the initramfs. It should automatically pick up any dependencies when you generate the initramfs. I had to do this for a process we use to image servers over the network. The Ubuntu documentation for initramfs tools should help get you started.

https://manpages.ubuntu.com/manpages/xenial/man8/initramfs-tools.8.html